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Visualizing the assisted migration argument

Formerly a taboo topic among conservationists, ‘assisted migration’ or ‘managed relocation’ – literally moving sensitive species to new habitats in order to save them – has recently started to come in for serious consideration. A paper out in PNAS this week offers a quick and innovative way to evaluate candidate species with new visual tools.

Some of the paper’s authors, including Stephen Schneider of Stanford, California and Jessica Hellman of Notre Dame, Indiana were part of a meeting last year on how to make such decisions, which we covered in a news feature at Nature Reports Climate Change.

The authors analyze the possibilities for three candidate species:


Image courtesy of PNAS.

Up for consideration at the top of list is the bay checkerspot butterfly, a Californian species whose native grass habitat is rapidly disappearing. While the butterfly could conceivably relocate northward, cities are currently in the way. But with a little help from humans, the checkerspot could bypass these obstables.

Next in line is Florida’s Torrey pine, which is already being moved northward by by amateur enthusiasts. (It’s practically a grassroots campaign! Oh, did I say that out loud? Sorry). And last but not least come commercial tree farms in Canada, where guidelines for planting in certain areas are already under review owing to anticipated climate impacts.

The various criteria that form the basis of these decisions are grouped under four headings. Focal Impact, plotted on the top axis, is a measure of how threatened the species is in its native land. Feasibility describes how easy it is to move the species; acceptability, societal views on whether it’s ok to move it, collateral impact is the effect expected in the new location. The latter is measured on a reverse scale, so that more color on every axis corresponds to a stronger argument for assisted migration. Shading shows confidence intervals, with medium hue representing the best estimate.

Each potential relocation has its advocates and opponents, and the side-by-side diagrams shown here capture hypothetical examples of these pro and con arguments. This is the key feature the authors say they are after: something that makes the reasoning of different stakeholders more transparent and comparable. That transparency is missing, they argue, from a decision tree proposed in a Science commentary from 2008.

Another big difference: the decision tree treats assisted migration is a last resort. The new paper tries to score how good an option it is without assuming it’s the worst possible choice.

Anna Barnett


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